Abstract: Agates commonly fill the bubbles of lava, but very few agate deposits developed in this way. The "Zhanguohong" agate from Xuanhua, Hebei Province, is a typical one that resulted from filling bubbles of lava. Detail study of it may help us to understand the metallogensis and/thus improve exploration for such kind of deposit. Our exploration recently revealed that the agates were hosted in the 4^th segment trachy lava of the middle Jurassic Tiaojishan Formation, which was covered by the 5^th segment volcanic breccia of the same formation. Its ore-bodies contain 0.7 to 1.2 kg agate per cubic meter. Microscopic observations demonstrated that a single filled bubble consists commonly of several bands of cryptocrystal SiO₂. Among the bands is a belt of fine-grained quartz or calcite. Both the cryptocrystal SiO₂-bands and the quartz belts contain numerous Fe-enriched and/or Mn-enriched dots. The change in content of the dot had led to the highly variable colour of the agate. Sythysizing ore-geology and microscopic studies, the authors suggest that the Shanggu agate deposit was resulted from rapid depositing of silicon in the bubbles of lava, and the associated SiO₂-enriched thermos-fluid likely came from the late-stage explosive volcanisim. Each rhythimical cryptocrystal silicon band represents a pulse of volcanism.